1. Technical Field of the Invention
The present invention relates to the chirped pulse amplification of an ultrashort optical pulse, and more specifically, to chirped pulse amplification using commercial telecommunications components, such as a LiNbO3 modulator for a pulse selector. The present invention further relates to techniques for integration of components together to avoid free space alignment, which results in a more simple assembly process and improved mechanical stability.
2. Description of the Related Art
The following references provide useful background information on the indicated topics, all of which relate to the invention, and are incorporated herein by reference:    A. Galvanauskas and M. E. Fermann, 13-W Average Power Ultrafast Fiber Laser, Conference on Lasers and Electro-Optics 2000, San Francisco, Calif., May 7-12, 2000, post deadline paper CPD3.    Y. Jaouen, M. Le Flohic, E. Olmedo and G. KUlsear, 35 kW Subpicosecond Pulse Generation At 1.55 um Using Er3+/Yb3+ Fiber Amplifier, Conference on Lasers and Electro-Optics 2001, Baltimore, Md., May 6-11, 2001, paper CTuQ3.    M. E. Fermann, M. L. Stock, A. Galvanauskas and D. J. Harter, HighPower Ultrafast Fiber Laser, Proceedings of SPIE, 3942, 194 (2000).    A. Boskovic, M. J. Guy, S. V. Chernikov, J. R. Taylor, and R. Kashyap, All-Fibre Diode Pumped Femtosecond Chirped Pulse Amplification System, Electronics Letters, 31 (11), 877 (1995).
With the increasing interest in high-pulse energy femtosecond lasers in applications such micro-structuring, the emergence of high power high pulse energy fiber lasers has been one of the most exciting developments in optical technology in recent years. Both Galvanauskas et al. and M. E. Fermann et al. have disclosed the achievement of microjoule levels of pulse energy in erbium and ytterbium-based chirped pulse amplification systems. However, at wavelengths of approximately 1550 nanometers, very little research has occurred recently. Researchers in the United Kingdom and France have done some work in this area, but at limited pulse energies. Jaouen et al. have used a peak power of 35 kilowatts and a pulsewidth of 450 femtoseconds, but the pulse energy was only 16 nanojoules. Boskovic et al. only obtained 1.6 nanojoules after amplification without down-counting the repetition rate from the source laser.
In most chirped pulse amplification systems, an acousto-optic (AO) modulator is used to select the pulses to be amplified. However, at wavelengths around 1550 nanometers, such an acousto-optic modulator is not readily available due to material limitations, especially when the original pulse repetition rate is higher than 20 megahertz. For example, if a mode-locked laser source with a pulse repetition rate of 50 megahertz is used, in order to select a pulse from the initial pulse train, less than 10 nanoseconds in rise time and fall time is normally required. However, at such a speed, acousto-optic modulators working at 1550 nanometers are either not readily available or very expensive. In addition, such modulators have high insertion losses. For example, Brimrose manufactures an AO modulator with acceptable performance at 1550 nanometers, but each modulator costs several thousand dollars. Such high costs can limit mass production of amplification systems using such AO modulators.
On the other hand, at 1550 nanometers, high speed electro-optic (EO) modulators (such as LiNbO3) working at 2.5 GHz/s and above (2.5 GHz/s, 10 GHz/s, even 40 GHz/s) are readily available and relatively cheap, due to the large inventory available in the telecommunications industry. A fiber pigtailed 2.5 GHz/s LiNbO3 modulator can be purchased for less than a thousand dollars. No chirped pulse amplification system, however, has ever used such an electro-optic modulator system.
A LiNbO3 electro-optic modulator is a type of Mach-Zehnder modulator. A LiNbO3 modulator comprises an integrated optical waveguide on a material that can exhibit electro-optic effects. Electro-optic materials have an index of refraction that can be changed with the application of voltage. Mach-Zehnder modulators operate using interferometry techniques. The optical signal is branched into two separate paths and is then recombined at the output. The two paths of the interferometer are nearly, but not exactly, the same length. When the two optical signals from the two paths are combined at the output, the two signals will have a slightly different phase. If these two signals are exactly in phase, then the light will combine in the output waveguide with low loss. However, if the two signals are 180° out of phase, the light will not propagate in the output waveguide and as a result, it will radiate into the surrounding substrate. The electro-optic effect makes the velocity of propagation in each path dependent on the voltage applied to the electrode. As a result, depending on the modulation voltage, the light will propagate with high or low loss at the output waveguide.
Commercial telecommunications modulators all have fiber pigtails aligned to the input and output waveguides. The input fiber pigtail has to be a polarization-maintaining fiber, since Mach-Zehnder modulators must have a specific input polarization state to function properly, But the output fiber pigtail can be either polarization-maintaining or non-polarization-maintaining fiber, depending on the application.
In a typical chirped pulse amplification system, a stretcher and one or two pre-amplifiers are needed, as well as the pulse selector before the power amplifier. The stretcher can be a bulk grating or fiber grating, or a fiber stretcher, as discussed in U.S. Pat. No. 5,847,863 issued to Galvanauskas et al., and hereby incorporated by reference in its entirety. However, even if a fiber-based device was used as stretcher, it was heretofore assembled using free space alignment, wherein a coupling element (e.g, a lens) coupled the input pulse into the fiber. Although technically sufficient, the coupling element is not suited to mass production, due to the labor-intensive assembly involved. In addition, the long-term operational stability of the system is usually an issue as well. For example, the coupling has to be frequently adjusted to ensure high throughput.
An erbium-doped fiber amplifier is a common active device, which uses a certain length of erbium-doped fiber and a pump diode (operating at either 980 nanometers or 1480 nanometers). Due to the non-polarization maintaining nature of the erbium-doped fiber, a double pass configuration has to be used to maintain the polarization. Due to the polarization sensitive nature of the 1200)3 modulator, it can not be used in the same double pass loop with other non-polarization-maintaining fiber components.
Normally, LiNb3 modulator has a low extinction ratio (˜23 decibels), which results in a low signal/noise contrast ratio, typically around 20-23 decibels. This low signal/noise contrast ratio is inadequate for a chirped pulse amplification system, and has to be increased to at least 30 decibels, or higher. In order to achieve this, either the polarization extinction ratio of the modulator must be improved, or other methods have to be exploited to increase the signal/noise contrast ratio.